Brief Answer:
?? Hot food, allergy, spicy food or infection from the given information

Detailed Answer:
Hello ma'am and welcome.

Thank you for writing to us.

I have gone through your query with diligence and would like you to know that I am here to help. Well ma'am, there are many causes for tongue swelling, especially on one side. I would thus like to start by listing all the causes, and then reaching the most likely cause in your case according to me, based on the provided information.

They are:

+Injury
+Infection
+Metabolic disorders
+Allergy
+Genetic disorders
+Thyroid abnormalities
+Chronic conditions
+Cancer
and a few others.

But due to the sudden and acute onset of your swelling, I am currently suspecting:

+Allergic reaction: Due to the appearance of symptoms after lunch (wherein the allergen can be located). Please go through what you ate, and what from that could have triggered an allergic reaction. I would also advise you to get your hands on some OTC anti-histamine and take it, if it helps then the cause is confirmed.

or

+Infection: This also could be due to some bacterial, viral, etc. infection

or

+Exposure to very hot (temperature), spicy food, alcohol or tobacco have also found to be some of the most common causes for tongue swelling.

I look forward to hearing from you in order to help you further. So please do not hesitate to write to me with any further clarifications, or information that can help me to help you. I hope you find my response both helpful and informative ma'am.

Best wishes.

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